Sibling is a correct word, but is used almost entirely in government / legal type papers... No one would ask someone if they had "siblings" or say that their siblings were coming to stay... I would reserve the use of this word only for legal forms, such as a passport application.
Yes, this is crazy. How can a plural- siblings- be a singular? The word itself takes a plural ending- moje , which suggests the plural. Another thing, talking about moje, I assume this is not a masculine plural by its ending but surely siblings could include males as well as females and so moi could be valid.
Okay, I accept that siblings has been decided as a singular noun in Polish. However, more generally can you remind me of the definition of a virile (masculine personal plural) noun, please? I may be wrong but I thought it was a group of people including in theory at least one male (and so siblings). Thanks.